A few 'newbee' questions...

[tubes4me]

Greetings,

I'm new here and was hoping someone could answer a few questions for me. I have an SCA-35 that I'm thinking about stripping for the iron.

First -- can someone compare how the diytube ST-35 compares soundwise to the original Dyna ST-35? I'm hoping it's very very close as the original sounds so special.

Second -- are there any plans to someday offer a board that provides for tube rectification?

Thanks!

[erichayes]

Hi All,

Thanks to Ed, I'll be completing restoration of an original original (old logo, pre-zipcode) ST-35 very shortly, and my intern will be finishing off a rev C soon after. My problem is output transformers again. as the Z-565 I had, along with the one Ed donated had to go to the restoration. I'll probably go with the Hammonds, but it would be nice to compare apples to apples (the ST-35 doesn't belong too me). I'll also be using my outputs on the rev C, and will publish the results here.

Tube rectification is generally better suited for off-board installation. The heat generated by a tube rectifier can play Hell with the tube socket connections, and the board substrate itself. Also, some people like to fool around with damper tube rectification. With only one exception I can think of, these tubes are half wave, thus necessetating 2 tube sockets. And then there are the guys who like miniature EZ81s in parallel...

The point is, the board provides an excellent platform for experimentation, whehter it's a version A or version D, but it's not a Heathkit. This forum, and its members, provide a safety net for neophytes to DIY tube construction (who, IMHO, are generally much faster learners than those of us more set in our ways). Ask questions, get answers, give answers, have fun.

[Shannon Parks]

Hi tubes4me & Eric,

When I first started this whole project back in 2001, I was given a hulker Knight receiver with PP EL84 outputs from my boss’s dad. They all smiled and said the thing had never worked too well. I tore it completely apart and wired it up as the Dynaco ST35 clone on the web (from the Aussie gent?) that just uses 12AX7s. At that point, I really started desiring a genuine ST35, but the cost seemed prohibitive. After purchasing my first SCA35 for around $100, I realized that any old amplifier basically needs total restoration - thus ballooning the costs of a genuine ST35 even higher. Couple that with my ineptitude running flying wires and the diytube ST35 was born.

I'd like to think that a well built original or my design would sound pretty much the same. I just received an email from Tom Fine a few weeks back telling me he swept an original that he built versus his diytube ST35 on an Audio Precision rig. They were identical within the uncertainty of the measurement.

I think it ultimately comes down to what you like to do. Some folks love restoration work. Some like building from scratch.

Eric, I plan to take a break from my low noise preamp testing and tweak the feedback network on the ST35 for the Hammond and your output. In my testing of the Z565 for Ned, I learned a little more about the characteristics of the Z565 (bandwidth sacrificed for low distortion). I'm guessing your output and the Hammond can use a much less conservative feedback arrangement. In addition, I may test a new A490 transformer from Ned that has a 9K primary which is in a 6B4G Ike on the test bench right now. Sorry Ned - I'm still working on this one!

Shannon

[EWBrown]

I have built up several DIY35s, from Rev B thru Rev D, and all sound great, absolutely no hum or noise. About 2 years ago, I obtained an NOS freebie unbuilt Dynaco ST35 kit, got it assembled and working, after carefully "reconditioning" the unused main electrolytic cap. This unit has a tiny bit of hum, so I would guess the 'new old' cap is the culprit. The DIY 35 has LOTS more capacitance, and a far better grounding scheme than the original, these both contribute to the lack of hum and noise.
Also, the original ST35 and SCA35 required a matched quad of 6BQ5s / EL84s, as they all shared a common 95 ohm cathode resistor. The DIY35 allows for individual tube biasing at 35 mA each, and each cathode has its own bypass cap to ground. The chief advantage to building up the DIY35 is that you can use 1% tolerance resistors and better quality coupling caps than the original had, these both contribute to the better sound quality.

One of the future '35 projects, I'll use tube rectification, the plan is to chassis mount one octal socket to accomodate a 5AR4/GZ34, and two 9 pin sockets, to try out two nice "guided missile grade" Bendix 6754 rectifier tubes These have 6.3V 1A filaments, so they may require an extra filament transformer - they each have two separate plates and cathodes, so they can be arranged in various configurations as needed.

http://www.mif.pg.gda.pl/homepages/fran ... 6/6754.pdf

Or perhaps some Bendix 5993s, similar but have 2 plates, 1 cathode.
http://www.mif.pg.gda.pl/homepages/fran ... 5/5993.pdf

/ed B in NH

[Tubes4me]

Thanks all for the responses. I'm pretty sure that I'll strip down the SCA-35 for the iron and build the DIY version. I would definitely want to go the tube rectification route (GZ34)...and, yes, it would be off board -- that's what I had in mind. I was just thinking that perhaps a newer version of the board could incorporate the necessary hookup points to make it more 'fool proof'. I don't know, maybe the existing board already makes it pretty easy...? Anyone how difficult it is with the existing board?

Thanks again!